Jumping in the Chicago River in the middle of winter may sound scary, not to mention freezing — but for “Chicago P.D.” stuntwoman Kelli Scarangello, it was a dream come true.

“It’s kind of a running joke here in Chicago that you’re not on official midwest stunt person until you go in the Chicago river — so I was excited,” she says in Variety‘s exclusive behind-the-scenes video below, which details the making of the major stunt airing in the Feb. 23 episode. The video was created by Wolf Entertainment.

In the midseason premiere, titled “Still Water,” Hailey Upton (Tracy Spiridakos) witnesses a car accident and jumps into the river to make a rescue. Throughout the episode, Upton’s investigative unit Intelligence must track down the man responsible, which leads to a second underwater scene — all of which created some of the most impressive stunt work the show has tackled.

Scarangello was the one to jump into the freezing cold water, which she remained in for about 25 seconds. In the shot, it appears to be Spiridakos, as she’s then shown underwater, swimming toward the submerged car. But she filmed her scenes in heated tanks in Los Angeles.

“I love the water and I’m a diver myself — I have an Open Water Diver certificate, but it was so long ago,” Spiridakos says. “It was great to just get reacquainted, and that was kind of the biggest thing,  just to familiarize myself with all of that.” She adds that she was trained specifically on how to open your eyes and breathe underwater: “When I first opened my eyes underwater, it stung and I kept inhaling water. Eventually, you get more comfortable underwater having your eyes open and coming up. I remember walking outside when we were in L.A. everything was just cloudy! That part was was a bit of a challenge.”

“Chicago P.D.” showrunner Gwen Sigan notes that the team informed Spiridakos about the episode early on to make sure she was comfortable with each step.

“It’s a big thing to ask, and to Tracy’s credit, she’s pretty fearless. When you give her something, she’s always excited to jump in and try it,” Sigan tells Variety. “She did training work in the pool. She worked on it quite a lot because it’s very physically demanding, what she did.”

The episode began production in December but had to pause due to omicron; it then picked back up in January. The entire hour was built around the opening river sequence and the day before filming was set to take place, the river froze over. The next morning, the crew had to take a barge out to remove blocks of ice, making sure it was safe to jump in.

“I can’t express how proud I am of everyone in this episode — the crew, the cast, they all really stepped up. They were asked to something challenging and I can’t put it to words,” co-executive producer and director Chad Saxton says, holding back tears. “It makes me get emotional because they did so well.”

“Chicago P.D.” is produced by Wolf Entertainment and Universal Television, a division of Universal Studio Group. New episodes air on NBC Wednesdays at 10 p.m. ET.